In the cable network example, the cable head end typically provides input signals to, for example, a set-top box. A multiplexing filter forms an input/output stage of the set-top box. The input signals, applied via a transmission line, may contain, for example, television signals. In a, so-called, Data Over Cable Service Interface Specification (DOCSIS) return channel, a transmitter generates an output signal at a frequency range of 5-54 MHz that is applied through the multiplexing filter.
Surges in transmission line voltages can change the operating range of the components and severely damage and/or destroy them. There are many sources which can cause harmful electrical energy surges. One source is a radio frequency (RF) interference that can be coupled to transmission lines from a multitude of sources. The transmission lines act as large antennas that may extend over several miles, thereby collecting a significant amount of RF noise power from such sources as radio broadcast antennas. Another source could be lightning. Therefore, it may be desirable to interpose an RF surge suppression device, forming direct current (DC) short or low impedance when a surge occurs.
Typical lightning waveforms are approximated by simpler waveforms described in the IEC 61000-4-5 document. Those waveforms have substantial low frequency content. The multiplexing filter at the input of the set-top box passes a substantial amount of energy in the low frequency range. Therefore, without surge protection, the multiplexing filter, disadvantageously, might transfer the energy from the surge to the DOCSIS transmitter output, exposing that transmitter to large voltages that could damage it. The same potential problem may be applicable also to a receiver or a transceiver, not shown, and it would also need to be protected.
A common practice for protecting consumer electronics is to place a surge protection device directly at the input of the set-top box to be protected. For example, a gas discharge tube will be placed at the RF connector input known as F connector. There is a minimum firing voltage that is necessary to meet and an appreciable length of time required for the protection device to turn on to provide the required protection. During such turn on delay time, the electronics might be exposed to large voltages. Those voltages can cause damage.
Using an additional, secondary protection that can be provided by a transient high speed diode also has a disadvantage. If such high speed diode is to be connected to an output terminal of, for example, the DOCSIS transmitter, the transmitter output signal itself might cause the diode to conduct, when an output signal of the transmitter is sufficiently large. The diode conduction might, disadvantageously, produce harmonics in the transmitted signal in normal operation. Harmonics in the transmitted signal are undesirable because they cause signal interference to other services. Therefore, it may be desirable to increase the isolation between the surge protection device and the transmitter output.